Process of producing low-sulphur pig iron from high-sulphur furnace burdens



Patented Apr. 21, 1925.

UNITED STATES 1,534,244 PATENT OFFICE.

SAMUEL PEACOCK, 0F WHEELING, WEST VIRGINIA, ASSIGN'OR '10 ALEC J.GERRARD,

' OF CHICAGO, ILLINOIS.

PROCESS OF PRODUCING LOW-SULPHUR PIG IRON FROM HIGH-SULPHUR FURNACEBURDENS.

No Drawing.

To all whom it may concern:

Be it known that I, SAMUEL Pnecoox, a citizen of the United States,residing at \Vlieelin in the county of Ohio and State of West Tirginia,have invented certain new and useful Improvements in Processes ofProducing Low-Sulphur Pig Iron from High-Sulphur Furnace Burdens; and Ido hereby declare the following to be a full, clear, and exactdescription of the invention, such as will enable others skilled in theart to which it appertains to make and use the same.

This invention relates 'to a process of making a low sulphur pig ironfrom a high sulphur furnace burden, and has for its object to accomplishthis result in'a manner more eflicient and less costly than has beenheretofore proposed.

With this and other objects in view, the invention'consists in the novelcombination of steps constituting the process all as will be more fullyhereinafter disclosed and particularly pointed out in the claims.

In order that the invention may bemore clearly understood, it is said Asis-well known, the modern iron-making blast furnace involves acounter-current heating effect, in which iron oxides are reduced bycarbon monoxide gas and solid carbon, and the non-ferrous components ofthe ore, fluxes and ash of the fuel are combined as fusible slags. It isessential that both the metal iron and slags thus produced take on andhold a free running molten condition in order that they may be separatedfrom each other in the hearth and be readily withdrawn from the furnaceas mobile liquids. Pure iron melts at, say 2800 F.-, and for thenecessary molten fluidity it must be heated to 3000 F., which latter isa temperature not directly obtainable by burning carbon with air. Theformation of iron by the reduction of iron oxide with carbon monoxide isstrongly exothermic, and when such reduction takes place with the oreand monoxide gas previously heated to 1600 F. to 1800 F., the heatliberated by the reaction is found to be sufficient to melt the metal,notwithstanding that at the instant of its formation it may exist assubstantially pure metallic iron. The molten iron thus formed, dripsthrough the fuel to the hearth, it contacts more or less with the fuelash Application filed March 14, 1924. Serial No. 699,314.

and white hot coke present as well as with the hot combustion gases, andit is partly reconverted to iron oxide by a reversion with the"carbonmonoxide present. It takes up carbon, silicon, etc., and produces as anultimate end-product a crude iron carbon compound having a melting pointabout 2500 F. and a high liquid mobility at about 2600 F. This product,known commercially as cast iron, has a number of well known uses, butits most important industrial ap plication is in the manufacture ofsteel, for

which purpose it is treated almost exclusively to remove wholl or inpart, the impurities unavoidably a ded to the substantially pure iron inthe blast furnace.

Unfortunately, the steel making methods,-

such as the Bessemer or open hearth treatments, require high temperaturereactions in which air is used to burn out the silicon,

carbon, etc. And in carrying out these methods, nitrogen, carbonmonoxide, metalloid oxycarbides, iron oxides and oxycarbides, hydrogen,nitrogen dissociation products, etc., are unavoidably more orlessintroduced into the metal. It therefore results that for steels ofstandard characteristics,. the metal must be further treated, usually inan electric furnace, to remove the ill effects of the first refiningstep.

If it were possible to disregard the sulphur and phosphorus present.in'theore and coke, afurnace burden could be proportioned for a slagcomposition of low free running temperature. The iron, it is true, wouldhave a high melting point, for it would be low in silicon and carbon,but still by using an excess of fuel and slag, the iron would besufficiently impure to maintain a tapping mobility in the furnacecrucible or hearth. But, unfortunately, the sulphur present in thecharge negatives such an operation. The furnace burden sulphur goes intothe furnace almost wholly as iron sulphide, and in the fuel.

The usual furnace practice for sulphur removal is to proportion thefluxes to make a highly basic, or what is known as a limey slag, on theassumption that a molten basic silicate in contact with iron sulphidewill effect a change over to the acid condition, making free iron andcalcium sulphide. But as calcium is electro positive to iron, such areaction cannot range beyond a very small 4 high temperatures used inmodern blast furdissolves in molten iron,

equilibrium balance, hence to remove a substantial quantity of sulphur alarge excess of such limey slag must be used, thus entailing increasedfuel costs and a lessened metal output per unit of plant investment. Thetemperature also must be forced to the highest point obtainable, tobring the limey slag to a Working fluidity.

In iron-making, it is well known that it is economically important toreduce the iron oxides in the upper part of the furnace, by means of thecarbon monoxide gases made at the tuyere zone. Carbon monoxide reducesiron oxide freely at 1000 F. But, however, if a limey slag must be usedin order to eliminate sulphur, the temperature at thetop of the boshmust reach or exceed 2600 F., and at this temperature thefree iron madeabove the bosh attacks carbon monoxide, producing iron oxide and freecarbon, thus:

Fe-l-CO FeO-f-C.

That is, iron already reduced is reconverted in the furnace to ironoxide, and such iron oxide in the furnace hearth reacts with white hotcoke to form iron carbide. It is at this point that the iron silicidsare formed.

While it is true that these conditions are desirable in making certaingrades of cast iron, yet for steel making it is obvious that energyis'being freely expended to no useful end.

Phosphorus is usually present in iron ores, fluxes, etc., as phosphatesof iron, limwnd aluminum. These phosphates have a comparatively lowmelting point and when fused prior to contact with red hot carbon, theyare reduced to phosphides. Iron phosphide but aluminum and calciumphosphides do not. These latter phosphides therefore react with ironoxides to form iron phosphides, but they do not react in contact withmolten iron only. At the naces,.iron oxides are always present, from thetop of the bosh to the tuyere zone, due to the high temperature reactionand reversion above mentioned; hence nearly all of the phosphorus in thefurnace burden ultimates in the hearth as a phosphide which dissolves inthe molten iron. The silicon and carbon forced into the iron is asomewhat simpler reaction, due to the conditions imposed on the sulphurelimination involving a very high hearth temperature, and avery hotslag.

This invention, on theother hand, avoids many of the foregoingobjections by proceeding as follows: To the furnace burden there isadded a quantity of sodium chloride NaGl of at least twice that requiredto chemically combine with all the sulphur and phosphorus present.

It thus results that even after a considerable proportion of said sodiumchloride has been lost throughvolatilization, there will still remain asufficient quantity to eliminate The slag, after the addition of thesaid sodium chloride, is found to be free running at a temperature at orbelow 2000 F., and the hearth temperature need not exceed 2400 F.Further the iron oxide of the charge is reduced to metallic iron in orabove the bosh, and the temperature is too low to cause a reversal ofthe reducing reaction, or a reproduction of iron oxide as in the priorprocedures above discussed. The sulphur and phosphorus present in theburden in contact with molten alkali chloride, being converted tosulphur and phosphorus chlorides, in accordance with the above equationsand both being volatile even at the temperature at the furnace throat,they readily escape with the combustion gases. Further the very fluidslag produced carries the iron sponge formed down to the furnace hearth.Although the alkali employed suffers more or less volatilization in theprocess, as above stated, Fyet as its boiling point is well above 1800 avery considerable portion of the alkali vapor condenses on therelatively cold burden a ove the bosh and is returned to the process,ultimating as a silicate and forming part of the slag. The reduced metalwill contain from 1.5% to 4.00% carbon, with or phoshigh grade of pigiron either free from silicon, phosphorus and sulphur, or containingonly traces or very small quantities of the same, notwithstanding thefact that a furnace burden high in sulphur was employed.

What is claimed is 1. The process of making pig iron low in its sulphurcontent from an iron ore having a high content of sulphur which consistsin adding to the usual furnace burden sodium chloride in a quantity ofat least twice that required to chemically combine with all the sulphurand phosphorus present; smelting said ore in the presence of saidchloride; and recovering the iron thus produced.

2. The process of making pig iron low in its sulphur content from aniron ore having a high content of. sul hur which consists in adding tothe usualurnace burden .sodium chloride in a quantit of at least twicethat required to chemical y combine with all the sulphur and phosphoruspresent; smelting said ore in the presence of said chloride at I, atemperature not substantially above 2400 pig iron from a high sulphuriron ore which consists in providing the usual blast furnace chargecontaining said ore to which has been added more than twice the quantityof sodium chloride than would be necessary to react with all the sulphurpresent; smelting said charge without permitting the hearth temperatureto greatly exceed 2400 F.; and discharging the molten slag and ironproduced in constant streams from said furnace. 20

SAMUEL PEACOCK.

